Continuing growth of computing power is fueled by a combination of architectural refinements and increased computer system operating frequency. While individual computer components benefit from increasing parallelism and decreasing clock periods, these individual components are ultimately limited by the buses which connect them. Accordingly, an increase in the operating frequency of a computer system bus can provide a great boon to total computer system performance.
Techniques which facilitate increasing bus frequencies are consequently likely to gain widespread acceptance. One such technique is the use of terminating resistors which couple the ends of bus signal lines to a terminating voltage. Such termination advantageously reduces signal line noise by eliminating signal distorting discontinuities formed by unterminated signal line ends.
While termination reduces signal line noise, terminating resistors cause additional power to be dissipated when the signal line is driven by a bus agent to a voltage other than the terminating voltage. The amount of power dissipated in the terminating resistors may be reduced by limiting the total voltage swing of the computer system bus. This not only limits the maximum voltage dissipated across a terminating resistor, but also allows more rapid signal switching.
Considering these advantages, a computer system utilizing terminating resistors often terminates the bus to a voltage less than the computer system voltage. In most cases, this terminating voltage is derived from the computer system voltage supply which typically fluctuates during normal operation. Due to the smaller voltage swing of the signal lines, fluctuations of the magnitude found in the computer system voltage supply may disrupt signaling if directly reflected in the terminating voltage.
A series of diodes which reduces the computer system voltage to the terminating voltage is one prior art terminating voltage supply which directly reflects such fluctuations. Forward biased diodes provide a voltage drop of the threshold voltage of the diode, thus an appropriate voltage drop from the computer system voltage to the terminating voltage may be obtained by the series coupling of several such diodes. Unfortunately, the fixed diode threshold voltage causes the termination voltage to directly reflect changes in the computer system voltage.
This may be problematic, for example, where a 3.3 volt supply allows 10% fluctuation and the terminating voltage is 1.5 volts. In this case, the permissible 0.3 volt fluctuation in the computer system voltage produces an intolerable 20% variation in the termination voltage. While a diode based solution may be cost effective, the resulting termination voltage variation may not be acceptable.
A commercially available voltage regulator chip is one prior art solution which can deliver a relatively stable voltage supply. These chips supply up to a certain amount of current within a given voltage range. While these chips typically overcome the problem of excessive voltage variation, voltage regulators capable of providing the large current demanded for termination in a high speed computer system may become prohibitively expensive for use in high volume.
Thus, the prior art demonstrates low cost voltage supplies with little power supply noise rejection as well as high cost power supplies delivering a precise output voltage. The prior art does not, however, provide a sufficiently inexpensive terminating voltage supply with adequate power supply noise rejection capabilities.